1. Field of the Invention
The present invention relates in general to polymer dispersions and liquid crystal composite films. More specifically, the present invention relates to a highly functional copolymer/liquid crystal dispersion formed by copolymerizing a hydrophilic monomer with a hydrophobic monomer to introduce hydrophobic nature, and adding a surfactant to the resultant copolymer.
2. Description of the Prior Art
As a medium for transmitting various kinds of information in the Age of Information, a liquid crystal display (hereinafter referred to as "LCD") is one of the most widely used apparatuses together with a cathode ray tube (referred to as "CRT"). However, LCDs adopting a twisted nematic (hereinafter referred to as "TN") or a super twisted nematic (referred to as "STN") mode which have been most widely used, are associated with light transmission problems and a complicated fabrication process thereof. This has made it difficult to produce large LCDs using these techniques.
Polymer dispersion liquid crystal (referred to as "PDLC") composite films have polymer and liquid crystal phases to form a composite film. These PDLC films control the scattering of transmitted light at the interface of the polymer matrix/liquid crystal phase, with liquid crystals responding to electrical energy. PDLC films are produced by dispersing low molecular weight nematic liquid crystals in the polymer matrix. The liquid crystal spheres have a diameter of 1 to 10 mm. The need for PDLC films started from using liquid crystals among new polymer material fields having electro-optical properties. PDLC composite films have several advantages compared with other liquid crystal display materials. PDLC composite films do not employ a polarizing plate, and therefore, there is more transmitted light, and a screen produced therefrom is much brighter. Furthermore, since there is no need to align the liquid crystals, the manufacturing process is comparatively simple and large plates can be prepared from PDLC films. The production cost of a PDLC device is less than that for a device using other materials. Moreover, the liquid crystals are dispersed in a polymer matrix, and the films are flexible, both factors contributing to stability.
The method for preparing PDLC composite films necessitates a phase separation procedure, so that low molecular weight nematic crystal liquids can be dispersed evenly in a polymer matrix. There are three phase separating methods, polymerization induced phase separation, thermally induced phase separation and solvent induced phase separation methods. In the present invention, a nematic curvilinear aligned phase (hereinafter referred to as "NCAP") method which allows for the formation of liquid crystal dispersion polymer composite film of PDLC film is employed, although it is not a phase separation method. The NCAP method refers to a method for preparing a PDLC shutter by dispersing and emulsifying a nematic liquid crystal capsule in a water-soluble polymer solution, applying the emulsion onto a conductive transparent plate, drying the emulsion to form a PDLC film, and then laminating another conductive transparent plate. The liquid crystal droplets are neither linked to each other nor dispersed in a regular size and show a very high contrast ratio.
In the prior art process for preparing a PDLC film, it was necessary to use a dispersing agent to improve the dispersibility of the liquid crystals in a hydrophilic polymer matrix. However, PDLC films prepared by the above methods have the following demerits. Chiefly, the response rate is slow. After removing an electrical field, it takes a long time for liquid crystals to be restored to their original position. In addition, the liquid crystals of these devices amount to about 80 to 85% by weight of the device, making the manufacturing cost too expensive.